Abstract:

There is provided a separation member for separating a first conveying
path formed by a first conveying member from a second conveying path
formed by a second conveying member. There is also provided an opposed
member facing a magnetic pole H4 at an upstream side of two magnetic
poles H4 and H6 that forms a developer-releasing magnetic pole H5.

Claims:

1. A developing device that stores therein a developer containing carrier
and toner and develops a latent image formed on an image carrier, the
developing device comprising:a developer carrier that is disposed
opposite to the image carrier and forms a plurality of magnetic poles
around the developer carrier;a developer regulating member that is
disposed below the developer carrier so as to oppose thereto and
regulates an amount of the developer carried on the developer carrier;
anda plurality of conveying members that forms a circulation route
through which the developer stored in the developing device is conveyed
along a longitudinal direction, whereinthe plurality of conveying members
includesa first conveying member that is opposite to the developer
carrier and supplies the developer to the developer carrier while
conveying the developer along the longitudinal direction, anda second
conveying member that is disposed above the first conveying member so as
to face the developer carrier, and conveys the developer released from
the developer carrier along the longitudinal direction,the developing
device further comprisesa separation member that is disposed in a
position opposite to the developer carrier, and separates a first
conveying path formed by the first conveying member from a second
conveying path formed by the second conveying member, whereinthe
developer carrier is formed in such a manner that a developer-releasing
magnetic pole for releasing the developer carried on the developer
carrier, among the plurality of magnetic poles, is sandwiched by two
magnetic poles having the same polarity, andan opposed member facing the
magnetic pole, of the two magnetic poles, formed at an upstream side of
the developer carrier in its rotation direction is disposed at the
upstream side of the developer carrier in the rotation direction while
the separation member is located opposite to the developer carrier.

2. The developing device according to claim 1, wherein the opposed member
is formed in such a manner that its opposed surface opposite to the
developer carrier becomes parallel to a virtual tangent line passing a
closest point on the developing roller to the opposed member when the
opposed member is viewed as a cross section orthogonal to a rotational
central axis of the developing roller.

3. The developing device according to claim 2, wherein the opposed member
is formed in such a manner that a gap between the opposed surface and the
closest point on the developer carrier becomes equivalent to a gap
between the developer regulating member and the developer carrier.

4. The developing device according to claim 1, wherein an edge portion of
the opposed member located at the downstream side of the developer
carrier in its rotation direction is extended toward an inner side of the
second conveying path.

5. The developing device according to claim 1, wherein an edge portion of
the opposed member located at the downstream side of the developer
carrier in its rotation direction is extended below a virtual horizontal
line passing a rotational central axis of the developer carrier when the
opposed member is viewed as a cross section orthogonal to the rotational
central axis of the developing roller.

6. The developing device according to claim 1, wherein the opposed member
is formed integrally with a developing case.

7. The developing device according to claim 1, wherein the opposed member
is a release assist roller for assisting release of the developer from
the developer carrier at a position of the developer-releasing magnetic
pole by causing a magnetic force to act on the developer carried on the
developer carrier.

8. The developing device according to claim 7, wherein the release assist
roller includesa sleeve rotating in a direction opposite to the rotation
direction of the developer carrier, anda magnet fixed to an inner side of
the sleeve and forming a plurality of magnetic poles around the sleeve,
whereinthe magnet of the release assist roller forms a first magnetic
pole, as one of the plurality of magnetic poles, having a polarity
different from that of the two magnetic poles of the developer carrier at
a position opposite to the developer carrier.

9. The developing device according to claim 8, wherein the magnet of the
release assist roller forms a developer-releasing magnetic pole for
releasing the developer carried on the release assist roller, as a
magnetic pole different from the first magnetic pole of the plurality of
magnetic poles, in such a manner that the developer-releasing magnetic
pole is sandwiched by two magnetic poles having the same polarity, at a
position opposite to the second conveying member.

10. The developing device according to claim 8, wherein the sleeve of the
release assist roller is made to rotate so that a linear velocity on its
outer periphery becomes a linear velocity ratio of 0.5 to 1 to a linear
velocity on an outer periphery of the developing roller.

11. The developing device according to claim 1, wherein the carrier is
formed so that its weight-average particle size becomes 20 to 60
micrometers.

12. A process cartridge detachably attached to an apparatus body of an
image forming apparatus, comprising:a developing device according to
claim 1; andthe image carrier, wherein the developing device and the
image carrier are formed integrally.

Description:

[0001]The present application claims priority to and incorporates by
reference the entire contents of Japanese Patent Application No.
2009-206536 filed in Japan on Sep. 8, 2009 and Japanese Patent
Application No. 2009-232964 filed in Japan on Oct. 7, 2009.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to an image forming apparatus such as
a copier, a printer, and a facsimile, or a multifunction product of these
devices using an electrophotographic system, and to a developing device
and a process cartridge provided therein. More particularly, the present
invention relates to a developing device in which at least two conveying
members among a plurality of conveying members that forms a circulation
route through which a developer is conveyed along the longitudinal
direction are arranged so as to face a developer carrier and in which a
developer regulating member for regulating an amount of developer at an
upstream side of a developing region is disposed below the developer
carrier, and to a process cartridge and an image forming apparatus.

[0004]2. Description of the Related Art

[0005]Conventionally, there has been known a technology for a developing
device, which stores a two-component developer containing toner and
carrier (which also includes a case of adding external additives or the
like) in an image forming apparatus such as a copier and a printer, and
in which at least two conveying members among a plurality of conveying
members for forming a circulation route through which a developer is
conveyed along the longitudinal direction are vertically arranged and in
which a developer regulating member for regulating an amount of developer
at an upstream side of a developing region is disposed below a developer
carrier (e.g., see Japanese Patent Application Laid-open No. H11-174810,
Japanese Patent Application Laid-open No. 2008-26408 and Japanese Patent
No. 3950735).

[0006]The developing device using the two-component developer has a toner
supply port provided at a part of the developing device, and toner is
appropriately supplied into the developing device through the toner
supply port depending on toner consumption in the developing device. The
supplied toner is stirred and mixed with the developer in the developing
device by a conveying member (a stirring and conveying member) such as a
conveying screw. Part of the stirred and mixed developer is supplied to a
developing roller (developer carrier). The developer carried on the
developing roller is controlled to an appropriate amount by a doctor
blade (developer regulating member) disposed below the developing roller,
and then, the toner in the two-component developer is caused to adhere to
a latent image formed on a photosensitive drum at an opposite position to
the photosensitive drum (image carrier). A magnet is fixed inside the
developing roller, and a plurality of magnetic poles is formed around the
developing roller by the magnet.

[0007]A first conveying member (a supplying screw) and a second conveying
member (a collecting screw) are arranged vertically in the developing
device disclosed in Japanese Patent Application Laid-open No. H11-174810
and the like. These two conveying members form a circulation route of the
developer. The first conveying member arranged in the lower side of the
developing device supplies the developer to the developing roller at a
position of a developer-scoop-up magnetic pole while conveying the
developer along the longitudinal direction. The second conveying member
arranged in the upper side of the developing device conveys the developer
released from the developing roller at a position of a
developer-releasing magnetic pole along the longitudinal direction (a
direction opposite to the conveying direction of the first conveying
member). The downstream side of a conveying path formed by the first
conveying member (a first conveying path) communicates with the upstream
side of a conveying path formed by the second conveying member (a second
conveying path) through a first relay portion. The developer having
reached the downstream side in the first conveying path stays at the
position and is pushed up to reach the upstream side in the second
conveying path. Here, the toner supply port is provided at an upstream
side in the second conveying path and new toner is appropriately supplied
through the toner supply port. The upstream side in the first conveying
path communicates with the downstream side in the second conveying path
through a second relay portion. The developer having reached the
downstream side in the second conveying path drops in the second relay
portion by its own weight, and is moved to the upstream side in the first
conveying path.

[0008]The developing device in which the conveying members are vertically
arranged in the above manner can be made small in the horizontal
direction as compared with a developing device in which a plurality of
conveying members is horizontally arranged (see, for example, FIG. 19 in
Japanese Patent Application Laid-open No. 2008-26408). Therefore, the
compact developing device is often used in tandem type color image
forming apparatuses in which a plurality of developing devices is
horizontally arranged. A developing device, in which the conveying
members are vertically arranged and a supplying route (the first
conveying path) of a developer to the developer carrier is separated from
a collecting route (the second conveying path) of a developer released
from the developer carrier, can reduce a concentration deviation in a
toner image formed on the image carrier because the developer after a
developing process is hard to be contained in a developer to be carried
on the developing roller and be supplied to the developing process, as
compared with the developing device in which the conveying members are
horizontally arranged (see, for example, FIG. 19 in Japanese Patent
Application Laid-open No. 2008-26408).

[0009]As explained above, an image forming apparatus that includes a
developing device with the doctor blade (developer regulating member)
disposed below the developing roller can reduce the length of a paper
conveying path from a paper feeding unit (paper storage) disposed in the
lower part of the image forming apparatus to a paper ejection tray, as
compared with an image forming apparatus that includes a developing
device (see, for example, FIG. 19 in Japanese Patent Application
Laid-open No. 2008-26408) with the doctor blade disposed above the
developing roller. Thus, first print time in the tandem type color image
forming apparatuses can be reduced (see, for example, FIG. 1 in Japanese
Patent Application Laid-open No. 2008-26408). Furthermore, because a
layout that the paper ejection tray is disposed in an upper side of the
image forming apparatus can be easily provided even if the paper
conveying path is made comparatively shorter, this layout is often used
in the tandem type color image forming apparatuses which are made small
in the horizontal direction.

[0010]In the developing devices disclosed in Japanese Patent Application
Laid-open No. H11-174810, Japanese Patent Application Laid-open No.
2008-26408 and Japanese Patent No. 3950735, the developer released from
the developing roller after the developing process at the position of the
developer-releasing magnetic pole may be again carried on the developing
roller in the second conveying path. Such an inconvenience becomes a
non-negligible problem especially in the developing device in which the
second conveying path (collecting route) is disposed above the first
conveying path (supplying route). Moreover, the inconvenience becomes
more significant when the fluidity of the developer decreases caused by
degradation of the developer over time.

[0011]Once the inconvenience occurs, the developer after the developing
process (developer in which the toner is consumed) is contained in a
developer which is to be carried on the developing roller and be supplied
to the developing process, and this causes concentration deviation to
occur in a toner image formed on the image carrier.

[0012]In order to suppress the inconvenience, there are possible measures
that provide a separation member for separating the first conveying path
from the second conveying path so as to face the developer carrier and
that optimize the position of the developer-releasing magnetic pole and
the position of the second conveying path. Furthermore, in order to
reliably release the developer from the developing roller at the position
of the developer-releasing magnetic pole, there are also possible
measures that set magnetic force of two magnetic poles having the same
polarity and forming the developer-releasing magnetic pole to be
sufficiently large.

[0013]In these cases, however, there is a possibility to cause such an
inconvenience that the developer released from the developing roller at
the position of the developer-releasing magnetic pole is again carried
(re-carried) on the developing roller by the magnetic force of an
upstream-side magnetic pole of the two magnetic poles that form the
developer-releasing magnetic pole. Once the inconvenience occurs, the
load of the developing roller at the position where the developer is
re-carried thereon becomes heavy, which causes rotational torque of the
developing roller to increase.

SUMMARY OF THE INVENTION

[0014]It is an object of the present invention to at least partially solve
the problems in the conventional technology.

[0015]According to an aspect of the present invention a developing device
stores therein a developer containing carrier and toner and develops a
latent image formed on an image carrier, the developing device comprises:
a developer carrier that is disposed opposite to the image carrier and
forms a plurality of magnetic poles around the developer carrier; a
developer regulating member that is disposed below the developer carrier
so as to oppose thereto and regulates an amount of the developer carried
on the developer carrier; and a plurality of conveying members that forms
a circulation route through which the developer stored in the developing
device is conveyed along a longitudinal direction, wherein the plurality
of conveying members includes a first conveying member that is opposite
to the developer carrier and supplies the developer to the developer
carrier while conveying the developer along the longitudinal direction,
and a second conveying member that is disposed above the first conveying
member so as to face the developer carrier, and conveys the developer
released from the developer carrier along the longitudinal direction, the
developing device further comprises a separation member that is disposed
in a position opposite to the developer carrier, and separates a first
conveying path formed by the first conveying member from a second
conveying path formed by the second conveying member, wherein the
developer carrier is formed in such a manner that a developer-releasing
magnetic pole for releasing the developer carried on the developer
carrier, among the plurality of magnetic poles, is sandwiched by two
magnetic poles having the same polarity, and an opposed member facing the
magnetic pole, of the two magnetic poles, formed at an upstream side of
the developer carrier in its rotation direction is disposed at the
upstream side of the developer carrier in the rotation direction while
the separation member is located opposite to the developer carrier.

[0016]The above and other objects, features, advantages and technical and
industrial significance of this invention will be better understood by
reading the following detailed description of presently preferred
embodiments of the invention, when considered in connection with the
accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is an explanatory diagram of an overall configuration
representing an image forming apparatus according to an embodiment of the
present invention;

[0018]FIG. 2 is an explanatory diagram of a configuration example (1) of
an imaging unit shown in FIG. 1;

[0019]FIG. 3(A) is a cross-sectional view representing a schematic
configuration when an upper portion of a developing device is viewed
longitudinally, and FIG. 3(B) is a cross-sectional view representing a
schematic configuration when a lower portion of the developing device is
viewed longitudinally;

[0020]FIG. 4 is a cross-sectional view representing a schematic
configuration when a circulation route of the developing device is viewed
longitudinally;

[0021]FIG. 5 is an explanatory diagram of a configuration example (1) of
the developing device shown in FIG. 2;

[0022]FIG. 6 is a schematic diagram of magnetic fields produced near a
separation member and an opposed member of the developing device shown in
FIG. 5;

[0023]FIG. 7A is a schematic diagram of a flow of a developer near a
developer-releasing magnetic pole when the opposed member is disposed in
the developing device shown in FIG. 5, and FIG. 7B is a schematic diagram
of a flow of the developer near the developer-releasing magnetic pole
when the opposed member is not disposed therein;

[0024]FIG. 8 is an explanatory diagram of a configuration example (2) of
the imaging unit shown in FIG. 1;

[0025]FIG. 9 is an explanatory diagram of a configuration example (2) of
the developing device shown in FIG. 8;

[0026]FIG. 10 is a schematic diagram of a flow of the developer near the
developer-releasing magnetic pole when a release assist roller is
disposed in the developing device shown in FIG. 9; and

[0027]FIG. 11 is a schematic diagram of a flow of the developer near the
developer-releasing magnetic pole when the release assist roller is not
disposed in the developing device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028]Exemplary embodiments to implement the present invention will be
explained in detail below.

First Embodiment

[0029]Embodiments to implement the present invention will be explained in
detail below with reference to the accompanying drawings. It should be
noted that same numerals are assigned to those the same as or
corresponding to portions in the figures, and thus explanation thereof is
appropriately simplified or omitted.

[0030]First, configuration and operation of an overall image forming
apparatus will be explained below with reference to FIG. 1.

[0031]In FIG. 1, 1 represents an apparatus body of a tandem type color
copier as the image forming apparatus; 3: an original feeding unit for
feeding an original to an original reading unit; 4: the original reading
unit for reading image information of the original; 5: a paper ejection
tray where an output image is stacked; 7: a paper feeding unit where a
recording medium P such as a transfer paper is stored; 9: a registration
roller for controlling a timing of conveying the recording medium P; 11Y,
11M, 11C, and 11BK: photosensitive drums as image carriers where toner
images of colors (yellow, magenta, cyan, and black) are formed
respectively; 13: developing devices for developing an electrostatic
latent image formed on each of the photosensitive drums 11Y, 11M, 11C,
and 11BK; and 14: transfer bias rollers (primary-transfer bias rollers)
for transferring the toner images formed on the photosensitive drums 11Y,
11M, 11C, and 11BK to the recording medium P in a superimposing manner.

[0032]Moreover, 17 represents an intermediate transfer belt to which the
toner images of the colors are superposedly transferred; 18: a
secondary-transfer bias roller for transferring a color toner image on
the intermediate transfer belt to the recording medium P; 20: a fixing
unit for fixing an unfixed image on the recording medium P; and 28: toner
containers of the colors for supplying toners (toner particles) of the
colors (yellow, magenta, cyan, and black) to the developing devices 13,
respectively.

[0033]The operation of forming an ordinary color image in the image
forming apparatus will be explained below. It should be noted that
imaging processes performed on the photosensitive drums 11Y, 11M, 11C,
and 11BK may be also referred to FIG. 2.

[0034]First, an original is fed from an original tray by a feeding roller
of the original feeding unit 3 and is set on a contact glass of the
original reading unit 4. Then, the original reading unit 4 optically
reads image information for the original set on the contact glass.

[0035]More specifically, the original reading unit 4 scans an image of the
original on the contact glass while irradiating the image with light
emitted from an illumination lamp. The light reflected by the original is
then formed on a color sensor through a mirror group and a lens. Color
image information of the original is read by the color sensor with each
of color-separated lights of RGB (red, green, and blue), and is
electrically converted into image signals. The color image information is
further subjected to processes such as a color conversion process, a
color correction process, and a spatial-frequency correction process in
an image processing unit based on the color-separated image signals of
RGB, to obtain the color image information for yellow, magenta, cyan, and
black.

[0036]The image information for the colors of yellow, magenta, cyan, and
black is transmitted to a writing unit (not shown). Laser lights L (see
FIG. 2) are emitted from the writing unit to the corresponding
photosensitive drums 11Y, 11M, 11C, and 11BK based on the image
information for the colors, respectively.

[0037]Meanwhile, the four photosensitive drums 11Y, 11M, 11C, and 11BK are
made to rotate in the clockwise direction of FIG. 1. Each of the surfaces
of the photosensitive drums 11Y, 11M, 11C, and 11BK is uniformly charged
at an opposed portion to a charging unit 12 (see FIG. 2) (which indicates
a charging process). A charging potential is thereby formed on each of
the photosensitive drums 11Y, 11M, 11C, and 11BK. Thereafter, each of the
charged surfaces of the photosensitive drums 11Y, 11M, 11C, and 11BK
reaches an irradiation position of corresponding laser light.

[0038]The writing unit emits laser lights corresponding to image signals
from four light sources, respectively, according to the colors. The laser
lights pass through different light paths for color components of the
yellow, magenta, cyan, and black, respectively (which indicates an
exposing process).

[0039]The laser light corresponding to the yellow component is irradiated
to the surface of the photosensitive drum 11Y at the first place from the
left side on the plane of paper. At this time, the laser light of the
yellow component is scanned in a rotation axis direction (main scanning
direction) of the photosensitive drum 11Y by a polygon mirror rotating at
a high speed. In this manner, an electrostatic latent image corresponding
to the yellow component is formed on the photosensitive drum 11Y after
being charged by the charging unit 12.

[0040]Likewise, the laser light corresponding to the magenta component is
irradiated to the surface of the photosensitive drum 11M at the second
place from the left side on the plane of paper, and an electrostatic
latent image corresponding to the magenta component is formed thereon.
The laser light corresponding to the cyan component is irradiated to the
surface of the photosensitive drum 11C at the third place from the left
side on the plane of paper, and an electrostatic latent image
corresponding to the cyan component is formed thereon. The laser light
corresponding to the black component is irradiated to the surface of the
photosensitive drum 11BK at the fourth place from the left side on the
plane of paper, and an electrostatic latent image corresponding to the
black component is formed thereon.

[0041]Thereafter, the surfaces of the photosensitive drums 11Y, 11M, 11C,
and 11BK where the electrostatic latent images of the colors are formed
reach opposed positions to the developing device 13, respectively. Then,
the toners of the colors are supplied from the developing devices 13 to
the photosensitive drums 11Y, 11M, 11C, and 11BK, respectively, and the
latent images on the photosensitive drums 11Y, 11M, 11C, and 11BK are
developed (which indicates a developing process).

[0042]Thereafter, the surfaces of the photosensitive drums 11Y, 11M, 11C,
and 11BK after the developing processes reach opposed portions to the
intermediate transfer belt 17, respectively. Here, the transfer bias
rollers 14 are disposed at the opposed portions so as to contact an inner
peripheral surface of the intermediate transfer belt 17. The toner images
of the colors formed on the photosensitive drums 11Y, 11M, 11C, and 11BK
are sequentially and superposedly transferred to the intermediate
transfer belt 17 at respective positions of the transfer bias rollers 14
(which indicates a primary transfer process).

[0043]Each of the surfaces of the photosensitive drums 11Y, 11M, 11C, and
11BK after the transfer processes reaches an opposed position to a
cleaning unit 15. The cleaning unit 15 collects non-transferred toner
remaining on each of the photosensitive drums 11Y, 11M, 11C, and 11BK
(which indicates a cleaning process).

[0044]Thereafter, each of the surfaces of the photosensitive drums 11Y,
11M, 11C, and 11BK passes through a decharging unit (not shown), so that
a series of imaging processes for the photosensitive drums 11Y, 11M, 11C,
and 11BK is finished.

[0045]On the other hand, the intermediate transfer belt 17 to which the
color toners on the photosensitive drums 11Y, 11M, 11C, and 11BK are
superposedly transferred (carried) moves in a counterclockwise direction
in the figure and reaches an opposed position to a secondary-transfer
bias roller 18. The color toner images carried on the intermediate
transfer belt 17 are then transferred to the recording medium P at the
opposed position to the secondary-transfer bias roller 18 (which
indicates a secondary transfer process).

[0046]Thereafter, the surface of the intermediate transfer belt 17 reaches
a position of an intermediate-transfer-belt cleaning unit (not shown).
The non-transferred toner depositing on the intermediate transfer belt 17
is collected by the intermediate-transfer-belt cleaning unit, so that a
series of transfer processes for the intermediate transfer belt 17 is
finished.

[0047]Here, the recording medium P conveyed to a space (which indicates a
secondary transfer nip) between the intermediate transfer belt 17 and the
secondary-transfer bias roller 18 is conveyed from the paper feeding unit
7 through the registration roller 9.

[0048]More specifically, the recording medium P fed from the paper feeding
unit 7 that stores the recording medium P by a paper feeding roller 8
passes through a conveying guide and is guided to the registration roller
9. The recording medium P having reached the registration roller 9 is
conveyed toward the secondary transfer nip at an appropriate timing.

[0049]The recording medium P with a full-color image transferred thereto
is then guided to the fixing device 20. In the fixing device 20, the
color image is fixed on the recording medium P at a nip between a fixing
roller and a pressing roller.

[0050]The recording medium P after being subjected to the fixing process
is ejected, as an output image, to the outside of the apparatus body 1 by
a paper ejection roller, is stacked on the paper ejection tray 5, and a
series of image forming processes is thereby completed.

[0051]Next, an imaging unit of the image forming apparatus will be
explained in detail below with reference to FIGS. 2 to 7.

[0052]FIG. 2 is a configuration diagram of the imaging unit. FIG. 3(A) is
a schematic cross-sectional view (horizontal cross-sectional view) when
an upper portion (where a second conveying screw 13b2 as the second
conveying member is disposed) of the developing device 13 is viewed
longitudinally, and FIG. 3(B) is a schematic cross-sectional view when a
lower portion (where a first conveying screw 13b1 as the first conveying
member is disposed) of the developing device 13 is viewed longitudinally.
FIG. 4 is a schematic cross-sectional view (vertical cross-sectional
view) when a circulation route of the developing device 13 is viewed
longitudinally. FIG. 5 is a cross-sectional view of the developing device
(cross-sectional view perpendicular to a rotational central shaft of a
developing roller 13a), and also represents a magnetic distribution of
components in a normal direction of magnetic poles H1 to H6 which are
formed on the developing roller 13a. FIG. 6 is a schematic diagram of
magnetic fields produced near a separation member 13d and an opposed
member 13k. FIG. 7A is a schematic diagram of a flow of a developer near
a developer-releasing magnetic pole H5 when the opposed member 13k is
disposed, and FIG. 7B is a schematic diagram of a flow of the developer
near the developer-releasing magnetic pole H5 when the opposed member 13k
is not disposed.

[0053]Because imaging units have almost the same configuration as one
another, the imaging units and the developing devices are shown without
alphabetical letters (Y, C, M, and BK) in FIGS. 2 to 7.

[0054]As shown in FIG. 2, the imaging unit includes the photosensitive
drum 11 as an image carrier, the charging unit 12, the developing device
13 (developing unit), and the cleaning unit 15.

[0055]The photosensitive drum 11 as the image carrier is a negatively
charged organic photosensitive element whose outer diameter is about 30
millimeters and is driven to rotate in the clockwise direction by a
rotation drive mechanism (not shown).

[0056]The charging unit 12 is an elastic charging roller with a foamed
urethane layer formed in a roller shape on its core bar. The foamed
urethane layer has medium resistance and is formed of materials including
urethane resin, carbon black as a conductive particle, a sulfating agent,
a foaming agent, and the like. The material of the medium-resistance
layer of the charging unit 12 that may be used includes a rubber material
in which a conductive substance such as carbon black and metal oxide is
dispersed into urethane, ethylene-propylene-diene polyethylene (EPDM),
butadiene acrylonitrile rubber (NBR), silicone rubber, and isoprene
rubber to adjust the resistance, and also includes a foamed material of
these.

[0057]The cleaning unit 15 includes a cleaning blade that is in sliding
contact with the photosensitive drum 11, so that the cleaning unit 15
mechanically removes and collects non-transferred toner on the
photosensitive drum 11.

[0058]In the developing device 13, the developing roller 13a as the
developer carrier is disposed so as to be close to the photosensitive
drum 11, and a developing region (a developing nip portion) where the
photosensitive drum 11 and a magnetic brush contact each other is formed
at opposed portions of the two. A developer G (two-component developer)
consisting of toner T and carrier C is stored in the developing device
13. In the present embodiment, 300 grams of the developer G in which
toner concentration is 7% by weight is stored in the developing device
13. The developing device 13 develops an electrostatic latent image
formed on the photosensitive drum 11 (to form a toner image). The
configuration and the operation of the developing device 13 will be
explained in detail later.

[0059]Referring to FIG. 1, the toner container 28 stores therein toner T
to be supplied into the developing device 13. Specifically, the toner T
is appropriately supplied from the toner container 28 toward inside the
developing device 13 through a toner supply port 13e via a toner
conveying tube (not shown) based on the information of the toner
concentration (a ratio of the toner in the developer G) detected by a
magnetic sensor (not shown) disposed in the developing device 13.

[0060]Information for supply of the toner T is not limited to the
information of the toner concentration, but the toner T may be supplied
based on the information of image density detected from reflectance and
the like of the toner image formed on the photosensitive drum, the
intermediate transfer belt, or the like. It may also be determined to
implement supply of the toner T based on a combination of these different
pieces of information.

[0061]The developing device 13 in the image forming apparatus will be
explained in detail below.

[0062]Referring to FIGS. 2 to 6, the developing device 13 includes the
developing roller 13a as the developer carrier, the conveying screws 13b1
and 13b2 (auger screws) as the conveying members, a doctor blade 13c as
the developer regulating member, the separation member 13d, the opposed
member 13k, and the like.

[0063]The developing roller 13a as the developer carrier is a developing
roller whose outer diameter is as small as about 18 millimeters, and it
is configured so that a sleeve 13a2 is made of a non-magnetic body such
as aluminum, brass, stainless steel, or conductive resin, which is formed
into a cylindrical shape, and is caused to rotate in the counterclockwise
direction by a rotation drive mechanism (not shown) at a speed of about
150 to 600 rpm. Referring to FIGS. 3 and 5, a magnet 13a1 forming a
plurality of magnetic poles H1 to H6 on the peripheral surface of the
sleeve 13a2 is fixed inside the sleeve 13a2 of the developing roller 13a.
The developer G carried on the developing roller 13a is conveyed along
with the rotation of the developing roller 13a in the direction of the
arrow, and reaches the position of the doctor blade 13c. The developer G
on the developing roller 13a is regulated to an appropriate amount with
the doctor blade 13c at this position, and then is conveyed to an opposed
position (a developing region) to the photosensitive drum 11. The toner
is attracted to the latent image formed on the photosensitive drum 11 by
an electric field (a developing electric field) produced in the
developing region.

[0064]FIG. 5 illustrates the plurality of magnetic poles H1 to H6 formed
around the developing roller 13a (the sleeve 13a2) by the magnet 13a1. As
illustrated in FIG. 5, the plurality of magnetic poles include a first
magnetic pole H1 (main magnetic pole) formed at an opposed position to
the photosensitive drum 11, a second magnetic pole H2 (conveying magnetic
pole) that is formed at a position on the downstream side of the first
magnetic pole H1 and in an upper portion of a developing case, a third
magnetic pole H3 (conveying magnetic pole) formed at the downstream side
of the second magnetic pole H2 and at the upstream side of a fourth
magnetic pole H4 (developer releasing pre-magnetic pole), the fourth
magnetic pole H4 (developer releasing pre-magnetic pole) formed at the
downstream side of the third magnetic pole H3, a fifth magnetic pole H5
(developer-releasing magnetic pole) formed at a position between the
fourth magnetic pole H4 and a sixth magnetic pole H6 and near the
separation member 13d, and the sixth magnetic pole H6 (developer-scoop-up
magnetic pole) formed over a region from an opposed position to the first
conveying screw 13b1 to an opposed position to the doctor blade 13c.

[0065]First, the sixth magnetic pole H6 (developer-scoop-up magnetic pole)
acts on the carrier as a magnetic body, and the developer G stored in the
first conveying path is fed onto the developing roller 13a. A part of the
developer G carried on the developing roller 13a is scraped off at the
position of the doctor blade 13c and is returned to the first conveying
path. On the other hand, the developer G, which is carried on the
developing roller 13a having passed through a doctor gap between the
doctor blade 13c and the developing roller 13a at the position of the
doctor blade 13c on which the magnetic force by the sixth magnetic pole
H6 acts, forms toner chains at the first magnetic pole H1 (main magnetic
pole) to become a magnetic brush in the developing region, and is brought
into a sliding contact with the photosensitive drum 11. In this manner,
the toner T in the developer G carried on the developing roller 13a
attaches to the latent image on the photosensitive drum 11. Thereafter,
the developer G having passed through the position of the first magnetic
pole H1 is conveyed to the position of the fifth magnetic pole H5
(developer-releasing magnetic pole) by the second magnetic pole H2, the
third magnetic pole H3, and the fourth magnetic pole H4. Then, a
repulsive magnetic field (which is a magnetic field acting on a direction
away from the developing roller 13a in FIG. 6) acts on the carrier at the
position of the developer-releasing magnetic pole H5, and the developer G
carried on the developing roller 13a after the developing process is
released from the developing roller 13a. The released developer G drops
into the second conveying path and is conveyed downstream in the second
conveying path by the second conveying screw 13b2.

[0066]Here, referring to FIG. 5, the six magnetic poles H1 to H6 are
formed of five poles (magnetic poles with "(N)" or "(S)" added to the
reference numerals in FIG. 5) magnetized by the magnet 13a1 of the
developing roller 13a. More specifically, among the six magnetic poles H1
to H6, only the fifth magnetic pole H5 (developer-releasing magnetic
pole) is not directly formed by the pole magnetized by the magnet 13a1,
but is formed in such a manner that it is sandwiched by two magnetic
poles (the fourth magnetic pole H4 and the sixth magnetic pole H6) having
the same pole (south (S) pole in the present embodiment).

[0067]Referring to FIG. 2 and the like, the doctor blade 13c as the
developer regulating member is a non-magnetic plate member (part thereof
can also be formed of a magnetic material) disposed below the developing
roller 13a. The developing roller 13a rotates in the counterclockwise
direction in FIG. 2, and the photosensitive drum 11 rotates in the
clockwise direction in FIG. 2.

[0068]This configuration allows the rotation direction of the developing
roller 13a to be set to a forward direction with respect to the
photosensitive drum 11 at the developing gap even if the photosensitive
drum 11 is disposed below the intermediate transfer belt 17 in order to
achieve shortening of the conveying path for the recording medium P and
horizontal downsizing of the apparatus body 1 of the image forming
apparatus. Therefore, the developing time at the developing gap may be
sufficiently ensured and the developing capability may be enhanced as
compared with a case where the doctor blade 13c is disposed above the
developing roller 13a and the rotation direction of the developing roller
13a with respect to the photosensitive drum 11 thereby becomes the
opposite direction.

[0069]The two conveying screws 13b1 and 13b2 (conveying members) stir and
mix the developer G while circulating the developer G stored in the
developing device 13 along the longitudinal direction (which is
orthogonal to the plane of paper in FIG. 2).

[0070]The first conveying screw 13b1 as the first conveying member is
arranged at a position opposite to the developing roller 13a. The first
conveying screw 13b1 horizontally conveys the developer G along the
longitudinal direction (the rotation axis direction) (to the left, as
shown by an arrowed dashed line in FIG. 3(B)), and supplies the developer
G onto the developing roller 13a at the position of the developer-scoop
-up magnetic pole H6 (sixth magnetic pole) (the supply to the direction
of white arrows in FIG. 3(B)). The first conveying screw 13b1 rotates in
the counterclockwise direction in FIG. 2.

[0071]The second conveying screw 13b2 as the second conveying member is
arranged at a position above the first conveying screw 13b1 and opposite
to the developing roller 13a. The second conveying screw 13b2
horizontally conveys the developer G released from the developing roller
13a (the developer G forcibly released from the developing roller 13a by
the developer releasing magnetic pole H5 after the developing process,
and is released to the direction of white arrows in FIG. 3(A)) along the
longitudinal direction (to the right, as shown by an arrowed broken line
in FIG. 3(A)). In the present embodiment, the rotation direction of the
second conveying screw 13b2 is set to be the opposite direction to the
rotation direction of the developing roller 13a (the clockwise direction
in FIG. 2).

[0072]The second conveying screw 13b2 conveys the developer G circulated
from the downstream side in the conveying path formed by the first
conveying screw 13b1 through a first relay portion 13f to the upstream
side in the conveying path formed by the first conveying screw 13b1
through a second relay portion 13g (in the arrow direction of dashed
one-dotted line in FIG. 3(B)).

[0073]The two conveying screws 13b1 and 13b2 are arranged so that the
rotational axes thereof become nearly horizontal, similarly to the
developing roller 13a and the photosensitive drum 11. Both the two
conveying screws 13b1 and 13b2 are formed in such a manner that a screw
portion whose outer diameter is about 20 millimeters (screw pitch: about
40 millimeters; number of screw threads: one or two screw threads) is
helically wound around a shaft portion whose shaft diameter is about 6 to
10 millimeters. The number of revolutions of the two conveying screws
13b1 and 13b2 is set to about 600 to 900 rpm.

[0074]The conveying path formed by the first conveying screw 13b1 (the
first conveying path) and the conveying path formed by the second
conveying screw 13b2 (the second conveying path) are isolated by a wall
portion.

[0075]Referring to FIGS. 3 and 4, the downstream side in the conveying
path formed by the second conveying screw 13b2 (second conveying path)
and the upstream side in the conveying path formed by the first conveying
screw 13b1 (first conveying path) communicates with each other through
the second relay portion 13g. The developer G having reached the
downstream side in the second conveying path formed by the second
conveying screw 13b2 drops by its own weight at the second relay portion
13g, to reach the upstream side in the first conveying path.

[0076]Referring to FIGS. 3 and 4, the downstream side in the conveying
path formed by the first conveying screw 13b1 and the upstream side in
the conveying path formed by the second conveying screw 13b2 communicate
with each other through the first relay portion 13f. The developer G,
which is not supplied onto the developing roller 13a through the first
conveying path formed by the first conveying screw 13b1, remains and
piles up near the first relay portion 13f, and is conveyed (supplied) to
the upstream side in the second conveying path formed by the second
conveying screw 13b2 through the first relay portion 13f.

[0077]In order to improve the conveying capability of the developer at the
first relay portion 13f ("gravity-defying" transfer of the developer from
the first conveying path to the second conveying path), a paddle-shaped
unit or a screw unit formed in such a manner that a winding direction of
a screw is opposite may also be provided at a position on the downstream
side in the first conveying screw 13b1 (the position corresponding to the
first relay portion 13f).

[0078]This configuration allows the two conveying screws 13b1 and 13b2 to
form the circulation route through which the developer G is
longitudinally circulated in the developing device 13. More specifically,
when the developing device 13 is operated, the developer G inside the
developing device 13 flows in the directions of the arrowed dashed lines
in FIGS. 3 and 4. In this manner, the concentration deviation of the
toner image formed on the photosensitive drum 11 may be reduced by
separating the supplying route of the developer G to the developing
roller 13a (the first conveying path formed by the first conveying screw
13b1) from the collecting route of the developer G released from the
developing roller 13a (the second conveying path formed by the second
conveying screw 13b2).

[0079]The magnetic sensor, which detects the toner concentration of the
developer circulating in the developing device 13, is disposed in the
conveying path formed by the second conveying screw 13b2 although it is
not shown in the figure. New toner T is supplied to the developing device
13 from the toner container 28 through the toner supply port 13e (which
is provided near the first relay portion 13f) based on the information
for the toner concentration detected by the magnetic sensor.

[0080]Referring to FIGS. 3 and 4, the toner supply port 13e is provided at
a position which is the upper side on the upstream side in the conveying
path formed by the second conveying screw 13b2 and apart from the
developing region (outside of the range along the longitudinal direction
of the developing roller 13a). By providing the toner supply port 13e
near the first relay portion 13f, the developer released from the
developing roller 13a falls down from above the supply toner that has low
specific gravity in the second conveying path, and the supply toner may
be sufficiently dispersed and mixed into the developer over a relatively
long time while moving toward the downstream side in the second conveying
path.

[0081]In the present embodiment, the toner supply port 13e is provided in
the developer conveying path formed by the second conveying screw 13b2,
however, the position of the toner supply port 13e is not limited
thereto. Therefore, for example, the toner supply port 13e may be
provided at the upper portion on the upstream side in the first conveying
path.

[0082]In addition, referring to FIG. 4, in the first conveying path, the
level of the developer G is getting low from the upstream side toward the
downstream side except for a portion near the first relay portion 13f in
order to supply the developer to the developing roller 13a while
conveying the developer along the longitudinal direction. On the other
hand, in the second conveying path, the level of the developer G is
getting high from the upstream side toward the downstream side because
the developer released from the developing roller 13a is collected while
conveying the developer along the longitudinal direction.

[0083]The developer G used in the present embodiment will be briefly
explained below.

[0084]The toner T (which includes the toner in the developer G and the
toner in the toner container 28) used in the present embodiment is
polymerized toner. As a binder resin, styrene-base resin (single polymer
or copolymer including styrene or styrene substitution) such as
styrene-acrylonitrile-acrylic acid ester copolymer, and polyester resin,
epoxy resin, or a compound of these may be used. As a method of
manufacturing these polymerized toners (polymerization method), bulk
polymerization, solution polymerization, emulsion polymerization,
suspension polymerization, or the like may be used.

[0085]As external additives of the toner T, non-organic particles (e.g.
those containing silica of 1.0% by weight and titanium oxide of 0.5% by
weight) are preferably used. Furthermore, as a releasing agent, oxidized
rice wax, low-molecular-weight polypropylene wax, carnauba wax, or the
like may be used. In addition, a charge controlling agent may be
contained in the releasing agent if necessary.

[0086]The toner T used in the present embodiment is formed so that its
volume-average particle size is 5.8 micrometers and toner particles whose
particle size is 5 micrometers or less are 60% to 80% by number.

[0087]The polymerized toner is used in the present embodiment, however,
pulverized toner may also be used.

[0088]The carrier C in the developer G used in the present embodiment is
formed so that its weight-average particle size is 20 to 60 micrometers.
In the present embodiment, the carrier C whose weight-average particle
size is 35 micrometers is used.

[0089]More specifically, the carrier C is formed so that methyl
methacrylate resin (MMA) whose film thickness is 0.5 millimeter is coated
on ferrite particles which become a core material and the above particle
size of the carrier C is thereby obtained. As the carrier C, coating
carrier with magnetite used as a core material may also be used.

[0090]By using such small sized carrier C, solid uniformity and halftone
image quality of an output image may be improved.

[0091]Specific configuration and operation of the developing device 13 in
the present embodiment will be explained below.

[0092]Referring to FIGS. 5 and 6, the separation member 13d (separating
plate) for separating the first conveying path from the second conveying
path is disposed at the position opposite to the developing roller 13a in
the developing device 13 according to the present embodiment. In other
words, there is the separation member 13d, which reduces the amount of
the developer G re-carried on the developing roller 13a after being
released from the developing roller 13a. The separation member 13d is
disposed at the position opposite to the developing roller 13a and
between the first conveying path and the second conveying path.

[0093]More specifically, the separation member 13d functions as a wall
portion for separating the first conveying path from the second conveying
path and is formed so as to protrude toward the developing roller 13a.
Moreover, the separation member 13d is formed integrally with a
developing case (a case member indicated by hatching in FIG. 2). The
separation member 13d is formed so that a gap between its opposed surface
opposite to the developing roller 13a and the developing roller 13a
becomes 2 millimeters or less (preferably 0.1 to 0.5 millimeter). In the
present embodiment, the gap between the separation member 13d and the
developing roller 13a is set to 0.3 millimeter.

[0094]Referring to FIG. 6, a magnetic field directed from the inner side
of the second conveying path toward the developing roller 13a (toward the
side of the first conveying path) is produced, caused by the influence of
the sixth magnetic pole H6, in a boundary region between the second
conveying path and the first conveying path. By disposing the separation
member 13d at the position where the magnetic field is blocked, it is
possible to prevent an inconvenience that the developer right after it is
collected in the second conveying path is again carried on the developing
roller 13a. Moreover, the separation member 13d is disposed so as to face
the developing roller 13a in a non-contact manner, which allows reduction
of the inconvenience that the surface of the developing roller 13a may be
damaged.

[0095]Because the separation member 13d is formed of a non-magnetic
material, an inconvenience that the carrier as the magnetic body is
magnetically attracted to the separation member 13d to block a flow of
the developer in the second conveying path or to promote a movement of
the developer to the first conveying path may be reduced.

[0096]Referring to FIG. 5, in the present embodiment, the second conveying
screw 13b2 (second conveying member) is disposed so that when it is
viewed as a cross section orthogonal to the rotational central axis of
the developing roller 13a (FIG. 5), the position of the rotational
central axis of the second conveying screw 13b2 is located below a
virtual horizontal line passing the upper edge of the developing roller
13a and above a virtual horizontal line passing the lower edge of the
developing roller 13a. More specifically, the position of the rotational
central axis of the second conveying screw 13b2 (or a virtual horizontal
line S2 passing the position) is provided within a range indicated by a
double-pointed arrow M1.

[0097]By configuring the present embodiment in this manner, not only a
magnetic force (outward magnetic force indicated by a dashed one-dotted
line in FIG. 6) works in a direction in which the developer G is strongly
released from the developing roller 13a; but also a resultant force of a
centrifugal force due to rotation of the developing roller 13a, a
pressure pushed by the developer at the downstream side, and of a gravity
or the like act on the developer G carried on the developing roller 13a
after the developing process, at the position of the developer-releasing
magnetic pole H5. With this feature, the developer G carried on the
developing roller 13a after the developing process is effectively
released from the developing roller 13a without release failure of the
developer from the developing roller 13a at the position of the
separation member 13d, and the released developer G is smoothly collected
to the second conveying path. Therefore, the developer G released from
the developing roller 13a after the developing process in the second
conveying path is hard to be re-carried on the developing roller 13a
right after the release, and thus the inconvenience that uneven image
density (concentration deviation) may occur on an output image is
reliably reduced.

[0098]Referring to FIG. 5, in the present embodiment, a peak magnetic
force of the first magnetic pole H1 is set to about 110 mT, a peak
magnetic force of the second magnetic pole H2: about 70 mT, a peak
magnetic force of the third magnetic pole H3: about 78 mT, a peak
magnetic force of the fourth magnetic pole H4: about 45 mT, and a peak
magnetic force of the sixth magnetic pole H6: about 65 mT.

[0099]The position of the developer-releasing magnetic pole H5 (fifth
magnetic pole) is variable by adjusting the positions (half-value center
angle) and the magnitudes (peak magnetic forces) of the fourth magnetic
pole H4 and the sixth magnetic pole H6. However, in order to reliably
prevent the developer from being carried together with a rotation of the
developing roller 13a (release failure) at the position of the
developer-releasing magnetic pole H5, the peak magnetic forces of the
fourth magnetic pole H4 and the sixth magnetic pole H6 are preferably set
to sufficiently large values which are equivalent to the above-mentioned
magnitudes.

[0100]Furthermore, referring to FIG. 5, in the present embodiment, the
separation member 13d is located at the position opposite to the
developing roller 13a, while the opposed member 13k (release assist
plate) facing the fourth magnetic pole H4 (which is a magnetic pole, of
the two magnetic poles H4 and H6 forming the developer-releasing magnetic
pole H5, formed at the upstream side of the developing roller 13a in its
rotation direction) is disposed at the position on the upstream side (on
the upstream side of the developing roller 13a in the rotation
direction).

[0101]More specifically, the opposed member 13k functions as a wall
portion for separating the position of the fourth magnetic pole H4 from
the second conveying path in the developing roller 13a. Moreover, the
opposed member 13k is formed integrally with the developing case (the
case member indicated by hatching in FIG. 2).

[0102]Referring to FIG. 6, a magnetic field directed toward the upstream
side of the developing roller 13a is produced caused by the influence of
the fourth magnetic pole H4 (developer releasing pre-magnetic pole) at
the upstream side of the developer-releasing magnetic pole H5 (the
repulsive magnetic field acting on a direction away from the developing
roller 13a in FIG. 6). By disposing the opposed member 13k at the
position that blocks the magnetic field, it is possible to prevent the
inconvenience that the developer released from the developing roller 13a
at the position of the developer-releasing magnetic pole H5 is again
carried (re-carried) on the developing roller 13a.

[0103]More specifically, as illustrated in FIG. 7B, if the opposed member
13k is not disposed, part of the developer (which moves in the direction
of white arrow) released from the developing roller 13a at the
developer-releasing magnetic pole H5 moves (moves in the direction of
black arrow) toward the upstream side of the developing roller 13a caused
by the magnetic field directed toward the upstream side of the developing
roller 13a due to the influence of the fourth magnetic pole H4 (developer
releasing pre-magnetic pole), and the part of the developer is thereby
re-carried on the developing roller 13a. Once such a phenomenon occurs,
the load of the developing roller 13a at the position where the developer
is re-carried becomes heavy, or the re-carried developer may not be fully
released from the developing roller 13a at the position of the
developer-releasing magnetic pole H5, which causes the developer to enter
into (or remain in) a gap between the separation member 13d and the
developing roller 13a, and rotational torque of the developing roller 13a
is thereby increased.

[0104]Meanwhile, in the present invention, as illustrated in FIG. 7A,
because the opposed member 13k is disposed at the position facing the
fourth magnetic pole H4 (developer releasing pre-magnetic pole), the
magnetic field directed toward the upstream side of the developing roller
13a due to the influence of the fourth magnetic pole H4 is blocked, and
it is thereby possible to reduce the inconvenience that the developer
released from the developing roller 13a at the position of the
developer-releasing magnetic pole H5 moves toward the upstream side of
the developing roller 13a to be re-carried on the developing roller 13a.
That is, the developer released from the developing roller 13a at the
position of the developer-releasing magnetic pole H5 smoothly moves in
the direction of white arrow in FIG. 7A.

[0105]Because the opposed member 13k is formed of a non-magnetic material,
the inconvenience that the carrier as the magnetic body is magnetically
attracted to the opposed member 13k to block a flow of the developer on
the developing roller 13a or to block a movement of the developer to the
second conveying path is reduced.

[0106]The inventors of this application conducted experiments. As a
result, it is ascertained that the developing device 13 (illustrated in
FIG. 7A) according to the present embodiment may reduce the rotational
torque of the developing roller 13a by about 15% as compared with the
developing device (illustrated in FIG. 7B) having no opposed member 13k.

[0107]Here, referring to FIG. 6 or the like, in the present embodiment,
the opposed member 13k is formed so that an opposed surface 13k1 opposite
to the developing roller 13a becomes nearly parallel to a virtual tangent
line S3 that passes a closest point Q on the developing roller 13a to the
opposed member 13k when the opposed member 13k is viewed as the cross
section orthogonal to the rotational central axis of the developing
roller 13a.

[0108]By configuring the present embodiment in this manner, the developer
carried on the developing roller 13a is smoothly conveyed toward the
position of the developer-releasing magnetic pole H5 without causing the
developer carried on the developing roller 13a to stagnate at the
position of the opposed member 13k.

[0109]Referring to FIG. 6 or the like, in the present embodiment, the
opposed member 13k is formed so that a gap CG between the opposed surface
13k1 and the closest point Q on the developing roller 13a becomes
equivalent to a gap (doctor gap) between the doctor blade 13c and the
developing roller 13a. Specifically, in the present embodiment, the gap
CG between the opposed member 13k and the developing roller 13a is set to
about 0.5 millimeter.

[0110]With this configuration, a height (height of toner chains) of the
developer carried on the developing roller 13a may be made equivalent to
a height (height of toner chains) of the developer (which passes through
the position of the doctor blade 13c) carried on the developing roller
13a after the amount of developer is regulated at the position of the
opposed member 13k. Therefore, the opposed member 13k may reduce an
inconvenience that adds stress to the developer carried on the developing
roller 13a.

[0111]Referring to FIG. 6 or the like, in the present embodiment, the
opposed member 13k is bent at a right angle so that a front end portion
(surrounded by a solid circle in FIG. 6) thereof located at the
downstream side of the developing roller 13a in the rotation direction is
extended toward the inside of the second conveying path.

[0112]With this configuration, it is possible to reliably block the
magnetic field directed toward the upstream side of the developing roller
13a due to the influence of the fourth magnetic pole H4, and it is also
possible to suppress an inconvenience that the developer released from
the developing roller 13a moves around to the back side (surface of the
opposite side of the opposed surface 13k1) of the opposed member 13k
caused by the magnetic field.

[0113]Referring to FIG. 5, FIG. 6, or the like, in the present embodiment,
the front end portion (surrounded by a solid circle in FIG. 6) of the
opposed member 13k located at the downstream side of the developing
roller 13a in the rotation direction is extended below a virtual
horizontal line S1 passing the rotational central axis of the developing
roller 13a.

[0114]With this configuration, the magnetic field directed toward the
upstream side of the developing roller 13a due to the influence of the
fourth magnetic pole H4 can be reliably blocked.

[0115]Here, referring to FIG. 5, in the present embodiment, the first
conveying screw 13b1 is provided so that the position of the rotational
central axis of the first conveying screw 13b1 is located at a position
(within a range M2 in FIG. 5) between a virtual vertical line passing the
rotational central axis of the second conveying screw 13b2 and a virtual
vertical line passing the rotational central axis of the developing
roller 13a when it is viewed as the cross section orthogonal to the
rotational central axis of the developing roller 13a.

[0116]This enables the first conveying screw 13b1 to be close to the
developer-scoop-up magnetic pole H6 of the developing roller 13a and
allows enhancement of supply performance of the developer G to the
developing roller 13a by the first conveying screw 13b1.

[0117]As explained above, in the present embodiment, when the two
conveying screws 13b1 and 13b2 (conveying members) that form the
circulation route through which the developer G is conveyed along the
longitudinal direction are arranged so as to face the developing roller
13a (developer carrier) and when the doctor blade 13c (developer
regulating member) is disposed below the developing roller 13a, the
separation member 13d is disposed opposite to the developing roller 13a,
thereby optimizing the position of the developer-releasing magnetic pole
H5 and the position of the second conveying path, and the opposed member
13k is disposed so as to face the fourth magnetic pole H4 at the upstream
side of the two magnetic poles H4 and H6 that form the
developer-releasing magnetic pole H5. Therefore, it is possible to
reliably reduce the inconvenience that the developer G released from the
developing roller 13a after the developing process in the second
conveying path is again carried on the developing roller 13a while
reducing an increase in rotational torque of the developing roller 13a.

[0118]In the present embodiment, the opposed member 13k and the separation
member 13d are formed integrally with the developing case, however, the
opposed member and the separation member may be formed of different
members from the developing case. More specifically, a plate-shaped
opposed member may also be hung from the developing case so as to face
the fourth magnetic pole H4 of the developing roller 13a. In addition, a
plate-shaped separation member may be adhered to the wall portion for
separating the first conveying path from the second conveying path in the
developing case. This configuration is useful for a case where although
component precision of the developing case cannot be made so much high,
the gap CG between the opposed member and the developing roller 13a or
the gap between the separation member and the developing roller 13a is
desired to be set highly precisely.

[0119]In the present embodiment, the toner T is supplied from the toner
container 28 to the developing device 13, however, the developer G (the
toner T and the carrier C) may also be supplied from the toner container
(developer container) to the developing device 13. This case requires a
unit for appropriately eliminating an excessive developer from the
developing device 13. Even in this case, the same effect as that of the
present embodiment may be obtained.

[0120]Also, in the present embodiment, the present invention is applied to
the image forming apparatus configured to form the developing device 13
as a single unit that is detachably attached to the body of the image
forming apparatus. However, the application of the present invention is
not limited thereto. Accordingly, the present invention can be applied
also to an image forming apparatus in which part of or the whole of the
imaging units is formed as a process cartridge. In this case, maintenance
workability of the imaging units may be improved.

[0121]Moreover, in the present embodiment, the present invention is
applied to the developing device 13 provided with the two conveying
screws as the conveying members, however, the present invention can be
also applied to a developing device in which at least two conveying
screws of three or more conveying screws are arranged so as to be
opposite to the developing roller 13a. In the present embodiment, the
number of magnetic poles H1 to H6 formed around the developing roller 13a
is set to six, however, the number of magnetic poles formed around the
developing roller 13a may also be set to five or less or seven or more.

[0122]Also, in these cases, by optimizing the position of the
developer-releasing magnetic pole and the position of the second
conveying path, the same effect as that of the present embodiment may be
obtained.

[0123]The present invention is not limited to the present embodiment, and
thus, it is obvious that the present embodiment may be changed if
necessary in addition to the indication in the present embodiment within
the scope of the technical idea of the present invention. The number,
positions, and forms or the like of the components are not limited to the
present embodiment, and thus, they may be set to those appropriate for
implementation of the present invention.

Second Embodiment

[0124]Next, explanation will be made on a developing device, different
from the developing device described above, provided with a release
assist roller, as an opposed member, for assisting release of a developer
from a developer carrier at a position of a developer-releasing magnetic
pole by causing the magnetic force to act on the developer carried on the
image carrier.

[0125]FIG. 8 is an explanatory diagram of a configuration example (2) of
the imaging unit shown in FIG. 1. FIG. 9 is an explanatory diagram of a
configuration example (2) of the developing device shown in FIG. 8.

[0126]The configuration of the image forming apparatus provided with the
developing device and the configuration of the circulation route i.e. of
the conveying screw as the conveying member are the same as those
illustrated in FIGS. 1, 3 and 4, thus omitting explanation thereof.

[0127]In the present embodiment, a release assist roller 23k is disposed
in the developing device 13 of the imaging unit shown in FIG. 8.

[0128]More specifically, as illustrated in FIG. 9, there is provided the
release assist roller 23k, at the position near the developer releasing
magnetic pole H24 and opposite to the developing roller 13a, for
assisting (promoting) release of the developer from the developing roller
13a at the position of the developer-releasing magnetic pole H24 by
causing the magnetic force to act on the developer carried on the
developing roller 13a. The configuration and the operation of the release
assist roller 23k will be explained in detail later.

[0129]In addition, the configuration and the operations of magnetic poles
of the developing device in FIG. 9 are the same as these of the first
embodiment. Thus, explanation of the arrangement and operations of the
magnetic poles and of a flow of the developer is omitted.

[0130]Specific configuration and operation of the developing device 13
according to the present embodiment will be explained below.

[0131]Referring to FIG. 9 and FIG. 10, in the developing device 13
according to the present embodiment, the separation member 13d is
disposed opposite to the developing roller 13a, while the release assist
roller 23k (developer releasing unit) is disposed at the position on the
upstream side of the developing roller 13a in the rotation direction and
opposite to the developing roller 13a near the third magnetic pole H23
(which is a magnetic pole, of the two magnetic poles H23 and H25 that
form the developer-releasing magnetic pole H24, formed at the upstream
side of the developing roller 13a in the rotation direction). The release
assist roller 23k is used to assist (promote) release of the developer
from the developing roller 13a at the position of the developer-releasing
magnetic pole H24 by causing the magnetic force to act on the developer
carried on the developing roller 13a.

[0132]More specifically, the release assist roller 23k is formed with a
sleeve 23k2 that rotates in the clockwise direction in FIG. 9 (the
opposite direction to the rotation direction of the developing roller
13a), a magnet 23k1 fixed inside the sleeve 23k2, and the like. The
sleeve 23k2 is formed of a non-magnetic metal material such as aluminum.
The magnet 23k1 forms a plurality of magnetic poles (four magnetic poles
R1 to R4 in the present embodiment) around the sleeve 23k2.

[0133]FIG. 10 illustrates the magnetic poles R1 to R4 formed around the
release assist roller 23k (sleeve 23k2) by the magnet 23k1. As shown in
FIG. 10, these magnetic poles include a first magnetic pole R1
(developer-attracting magnetic pole) formed at the position opposite to
the developing roller 13a, a second magnetic pole R2 (developer releasing
pre-magnetic pole) that is formed at the downstream side of the first
magnetic pole R1, a third magnetic pole R3 (developer-releasing magnetic
pole) formed at the position between the second magnetic pole R2 and a
fourth magnetic pole R4 and opposite to the second conveying screw 13b2,
and the fourth magnetic pole R4 (post-developer releasing magnetic pole)
formed at the upstream side of the first magnetic pole R1.

[0134]First, the first magnetic pole R1 (developer-attracting magnetic
pole) acts on the developer (carrier) carried on the developing roller
13a opposite thereto, so that part of the developer carried on the
developing roller 13a is released from the developing roller 13a and is
attracted to the release assist roller 23k (or, a retention force of the
developer on the developing roller 13a is weakened). The developer
carried on the release assist roller 23k is conveyed to the position of
the third magnetic pole R3 (developer-releasing magnetic pole) by the
second magnetic pole R2. The repulsive magnetic field (which is a
magnetic field acting on a direction away from the release assist roller
23k) acts on the carrier at the position of the developer-releasing
magnetic pole R3, so that the developer G carried on the release assist
roller 23k after the developing process is released from the release
assist roller 23k (which indicates a flow of the developer in the
direction of black arrow in FIG. 10). The developer G after being
released drops in the second conveying path and is conveyed toward the
downstream in the second conveying path by the second conveying screw
13b2. The developer not being attracted to the release assist roller 23k
and retained on the developing roller 13a is successfully released from
the developing roller 13a at the position of the developer-releasing
magnetic pole H24 of the developing roller 13a after the retention force
is weakened by the magnetic force of the first magnetic pole R1 (which
indicates a flow of the developer in the direction of white arrow in FIG.
10).

[0135]Here, referring to FIG. 10, the four magnetic poles R1 to R4 are
formed of three poles (magnetic poles with "(N)" or "(S)" added to the
reference numerals in FIG. 10) magnetized by the magnet 23k1 of the
release assist roller 23k. More specifically, among the four magnetic
poles R1 to R4, only the third magnetic pole R3 (developer-releasing
magnetic pole) is not directly formed by the pole magnetized by the
magnet 23k1, but is formed in such a manner that it is sandwiched by the
two magnetic poles (the second magnetic pole R2 and the fourth magnetic
pole R4) being the same pole (north (N) pole in the present embodiment).

[0136]The first magnetic pole R1 (developer-attracting magnetic pole) of
the release assist roller 23k is formed so that the polarity thereof is
different from the polarity of the two magnetic poles H23 and H25 on the
developing roller 13a that form the developer-releasing magnetic pole
H24. More specifically, the polarity of the third magnetic pole H23 and
the fifth magnetic pole H25 on the developing roller 13a is the N pole,
while the polarity of the first magnetic pole R1 (developer-attracting
magnetic pole) is set to the S pole. This allows smooth release of the
developer from the developing roller 13a at the position of the
developer-releasing magnetic pole H24.

[0137]Here, in the present embodiment, the release assist roller 23k is
set in such a manner that the outer diameter of the sleeve 23k2 is 10
millimeters, the outer diameter of the magnet 23k1 is 9 millimeters, and
a gap with the developing roller 13a is 2 millimeters.

[0138]The sleeve 23k2 of the release assist roller 23k is made to rotate
so that its linear velocity on its outer periphery becomes a linear
velocity ratio of 0.5 to 1 to a linear velocity on the outer periphery of
the developing roller 13a. More specifically, in the present embodiment,
the sleeve 23k2 of the release assist roller 23k is set so that the
linear velocity on the outer periphery becomes 250 mm/sec.

[0139]With this configuration, the release of the developer from the
developing roller 13a is smoothly performed at the position of the
developer-releasing magnetic pole H24. More specifically, the release
failure of the developer from the developing roller 13a at the position
of the developer-releasing magnetic pole H24 is reduced, and the
inconvenience that the developer released from the position of the
developing roller 13a after the developing process in the second
conveying path is re-carried on the developing roller 13a is hard to
occur.

[0140]Referring to FIG. 9 and FIG. 10, the separation member 13d for
separating the first conveying path from the second conveying path is
disposed at the position opposite to the developing roller 13a in the
developing device 13 according to the present embodiment. In other words,
there is the separation member 13d, which reduces the amount of the
developer G released from the developing roller 13a and re-carried on the
developing roller 13a, disposed at the position opposite to the
developing roller 13a and between the first conveying path and the second
conveying path.

[0141]More specifically, the separation member 13d functions as the wall
portion that separates the first conveying path from the second conveying
path, and is formed so as to protrude toward the developing roller 13a.
Moreover, the separation member 13d is formed integrally with the
developing case (which is the case member indicated by hatching in FIG.
8). The separation member 13d is formed so that a gap between its opposed
surface opposite to the developing roller 13a and the developing roller
13a becomes 2 millimeters or less (preferably 0.1 to 0.5 millimeter). In
the present embodiment, the gap between the separation member 13d and the
developing roller 13a is set to 0.3 millimeter.

[0142]Produced in a boundary region between the second conveying path and
the first conveying path is a magnetic field directed from the inner side
of the second conveying path toward the developing roller 13a (toward the
side of the first conveying path) caused by the influence of the fifth
magnetic pole H25. By disposing the separation member 13d at the position
where the magnetic field is blocked, it is possible to prevent the
inconvenience that the developer right after it is collected in the
second conveying path is again carried on the developing roller 13a.
Moreover, the separation member 13d is arranged so as to face the
developing roller 13a in a non-contact manner, which allows reduction of
the inconvenience that the surface of the developing roller 13a may be
damaged.

[0143]Because the separation member 13d is formed of a non-magnetic
material, the inconvenience that the carrier as the magnetic body is
magnetically attracted to the separation member 13d to block a flow of
the developer in the second conveying path or to promote a movement of
the developer to the first conveying path may be reduced.

[0144]Referring to FIG. 9, in the present embodiment, the second conveying
screw 13b2 (second conveying member) is arranged so that when it is
viewed as a cross section orthogonal to the rotational central axis of
the developing roller 13a (FIG. 9), the position of the rotational
central axis of the second conveying screw 13b2 is located below a
virtual horizontal line passing the upper edge of the developing roller
13a and above a virtual horizontal line passing the lower edge of the
developing roller 13a. More specifically, the position of the rotational
central axis of the second conveying screw 13b2 (or the virtual
horizontal line S2 passing the position) is provided within the range
indicated by the double-pointed arrow M1.

[0145]By configuring the present embodiment in this manner, a force with
which the developer G is strongly released from the developing roller 13a
is acted on the developer G carried on the developing roller 13a after
the developing process by the release assist roller 23k at the position
on the upstream side of the developer-releasing magnetic pole H24, and a
resultant force of a magnetic force that works in a direction in which
the developer G is strongly released from the developing roller 13a, a
centrifugal force due to rotation of the developing roller 13a, a
pressure pushed by the developer at the downstream side, and of gravity
acts on the developer G carried on the developing roller 13a after the
developing process at the position of the developer-releasing magnetic
pole H24. With this feature, the developer G carried on the developing
roller 13a after the developing process is effectively released from the
developing roller 13a over a range from the position opposite to the
release assist roller 23k to the position opposite to the separation
member 13d without release failure of the developer from the developing
roller 13a, and the released developer G is smoothly collected to the
second conveying path. Therefore, the release failure of the developer G
from the developing roller 13a is hard to occur, and the developer G
released from the developing roller 13a after the developing process in
the second conveying path right is hard to be re-carried on the
developing roller 13a right after the release, which allows reliable
reduction of the inconvenience that uneven image density (concentration
deviation) may occur on an output image.

[0146]Meanwhile, as shown in FIG. 11, if the release assist roller 23k is
not provided, part of the developer (which moves in the direction of
white arrow) released from the developing roller 13a at the position of
the developer-releasing magnetic pole H24 moves to the upstream side of
the developing roller 13a (moves in the direction of bold black arrow)
caused by the magnetic field directed toward the upstream side of the
developing roller 13a due to the influence of the fourth magnetic pole
H24 (developer releasing magnetic pole), and the part of the developer is
thereby re-carried on the developing roller 13a. Once such a phenomenon
occurs, the load of the developing roller 13a at the position where the
developer is re-carried becomes heavy, or the re-carried developer may
not be fully released from the developing roller 13a at the position of
the developer-releasing magnetic pole H24, which causes the developer to
enter into (or remain in) a gap between the separation member 13d and the
developing roller 13a, and rotational torque of the developing roller 13a
is thereby increased.

[0147]On the other hand, in the present embodiment, as illustrated in FIG.
10, because the release assist roller 23k is disposed at the position
facing the third magnetic pole H23 (developer releasing pre-magnetic
pole), it is possible to reduce the inconvenience that the developer
released from the developing roller 13a at the position of the
developer-releasing magnetic pole H24 moves toward the upstream side of
the developing roller 13a to be re-carried on the developing roller 13a
because the developer is attracted to the release assist roller 23k, so
that the magnetic field directed toward the upstream side of the
developing roller 13a due to the influence of the third magnetic pole H23
is overcome. That is, the developer released from the developing roller
13a at the position of the developer-releasing magnetic pole H24 smoothly
moves in the direction of white arrow in FIG. 10.

[0148]Here, referring to FIG. 9, in the present embodiment, the first
conveying screw 13b1 is provided so that the position of the rotational
central axis of the first conveying screw 13b1 is located at a position
(within a range M2 in FIG. 9) between a virtual vertical line passing the
rotational central axis of the second conveying screw 13b2 and a virtual
vertical line passing the rotational central axis of the developing
roller 13a when it is viewed as the cross section orthogonal to the
rotational central axis of the developing roller 13a.

[0149]This enables the first conveying screw 13b1 to be close to the
developer-scoop-up magnetic pole H26 of the developing roller 13a and
allows enhancement of supply performance of the developer G to the
developing roller 13a by the first conveying screw 13b1.

[0150]As explained above, in the present embodiment, when the two
conveying screws 13b1 and 13b2 (conveying members) that form the
circulation route through which the developer G is conveyed along the
longitudinal direction are arranged so as to face the developing roller
13a (developer carrier), and when the doctor blade 13c (developer
regulating member) is disposed below the developing roller 13a, the
separation member 13d is disposed opposite to the developing roller 13a,
and the release assist roller 23k is disposed near the third magnetic
pole H23 at the upstream side of the two magnetic poles H23 and H25 that
form the developer-releasing magnetic pole H24. Therefore, the developer
G is successfully released from the developing roller 13a at the position
of the developer-releasing magnetic pole H24, and thus, it is hard to
cause an inconvenience that the developer G released from the developing
roller 13a after the developing process in the second conveying path is
again carried on the developing roller 13a. Especially, even if the
fluidity of the developer is decreased caused by degradation of the
developer over time, the configuration according to the present
embodiment allows the developer G to be successfully released from the
developing roller 13a at the position of the developer-releasing magnetic
pole H24, and thus, it is hard to cause the inconvenience that the
developer G released from the developing roller 13a after the developing
process in the second conveying path is again carried on the developing
roller 13a.

[0151]In the present embodiment, the opposed member 13k and the separation
member 13d are formed integrally with the developing case, however, the
separation member and the developing case can be formed of separate
members. More specifically, a plate-shaped separation member can be
adhered to the wall portion for separating the first conveying path from
the second conveying path in the developing case. This configuration is
useful for a case where although component precision of the developing
case cannot be made so much high, the gap between the separation member
and the developing roller 13a is desired to be set highly precisely.

[0152]In the present embodiment, the toner T is supplied from the toner
container 28 to the developing device 13, however, the developer G (the
toner T and the carrier C) can also be supplied from the toner container
(developer container) to the developing device 13. This case requires a
unit for appropriately eliminating an excessive developer from the
developing device 13. Even in this case, the same effect as that of the
present embodiment can be obtained.

[0153]Also, in the present embodiment, the present invention is applied to
the image forming apparatus configured to form the developing device 13
as a single unit that is detachably attached to the body of the image
forming apparatus. However, the application of the present invention is
not limited thereto. Accordingly, the present invention can be applied
also to an image forming apparatus in which part of or the whole of the
imaging units is formed as a process cartridge. In this case, maintenance
workability of the imaging units can be improved.

[0154]Moreover, in the present embodiment, the present invention is
applied to the developing device 13 provided with the two conveying
screws as the conveying members, however, the present invention can be
also applied to a developing device in which at least two conveying
screws of three or more conveying screws are arranged so as to be
opposite to the developing roller 13a. In the present embodiment, the
number of magnetic poles H21 to H26 formed around the developing roller
13a is set to six, however, the number of magnetic poles formed around
the developing roller 13a can also be set to five or less or 7 or more.
Furthermore, in the present embodiment, the number of magnetic poles R1
to R4 formed around the release assist roller 23k is four, however, the
number of magnetic poles formed around the release assist roller 23k may
be set to three or less or five or more.

[0155]Also, in these cases, by disposing the release assist roller 23k
near the developer-releasing magnetic pole, the same effect as that of
the present embodiment can be obtained.

[0156]According to the present invention, when at least two conveying
members of the conveying members that form the circulation route through
which the developer is conveyed along the longitudinal direction are
arranged so as to be opposite to the developer carrier, and when the
developer regulating member is disposed below the developer carrier, the
separation member is disposed opposite to the developer carrier, and the
opposed member facing the upstream-side magnetic pole of the two magnetic
poles that form the developer-releasing magnetic pole is also provided.
Therefore, it is possible to provide the developing device, the process
cartridge, and the image forming apparatus capable of suppressing that
the developer released from the developer carrier after the developing
process in the second conveying path is again carried on the developer
carrier while reducing an increase in rotational torque of the developer
carrier.

[0157]Although the invention has been described with respect to specific
embodiments for a complete and clear disclosure, the appended claims are
not to be thus limited but are to be construed as embodying all
modifications and alternative constructions that may occur to one skilled
in the art that fairly fall within the basic teaching herein set forth.